Behind Ford's lithium-ion decision

When Ford Motor Co. announced in 2012 that its new hybrids would use
lithium-ion batteries instead of nickel-metal hydride, many experts
raised an eyebrow.

Lithium-ion, after all, had a reputation for high cost and unknown
durability, largely because the technology was still comparatively
new. In contrast, approximately 95 percent of full and mild
hybrids up to that time had used nickel-metal hydride.

But Ford engineers now say their decision to use lithium-ion was
based on accelerated lab tests showing lithium-ion would actually
be more durable than nickel-metal hydride over a long
lifetime. The tests, combined with mountains of field performance
data on nickel-metal hydride, convinced them that they could
predict the eight- or 10-year future of a chemistry that didn't
even have five years worth of reliable field data.

"We are really confident that our Key Life Tests are mimicking
the duty cycle of some of our most stringent and abusive
customers," Kevin Layden, Ford's director of electrification
programs and engineering, told Design News. "Given that, we feel
lithium-ion will be better than nickel-metal hydride. We expect it
to be absolutely stellar."

Ford's
confidence in lithium-ion is based on so-called Key Life
Tests. The tests predict that the working capacity (y-axis)
of lithium-ion batteries (green line) will be greater over a
high-mileage lifetime (x-axis) than that of nickel-metal
hydride (yellow line). Past field data for nickel-metal
hydride (blue dots) has shown that the testing results are
conservative -- that is, batteries generally do better in
the field than they do on tests.(Source: Ford Motor Co.)

My observation of lithium batteries is that a signficant portion of their wear out comes even without cycling. In other words, they have a independant time factor of degradation in addition to that associated with harsh use conditions.